Support is requested for a Keystone Symposia meeting entitled T Cells: Regulation and Effector Function, organized by Alexander Y. Rudensky, Dan R. Littman and Kristin A. Hogquist. The meeting will be held in Snowbird, Utah from March 29 - April 3, 2015. T lymphocytes play a pivotal role in the immune system of vertebrate animals as a key regulatory and effector cell type. Recently, remarkable progress has been made in the understanding of molecular and cellular mechanisms of differentiation and activation of T lymphocytes and in the elucidation of diverse functional T cell subsets and their biological roles in a variety of physiological and pathological settings. This Keystone Symposia meeting will bring together leading investigators from academia and industry studying diverse aspects of T cell biology. The overarching goal of this broadly themed meeting is to facilitate the development of a synthetic view of increasingly complex mechanisms of T cell differentiation and function and of interactions between diverse types of T lymphocytes and other cells of the immune system in health and disease. By attracting a diverse array of scientists and students this integrative meeting will cover a wide array of topics and stimulate discussions and interactions, and foster collaborations. The research that will be discussed at this conference is directly related to NIAID's mission to understand, treat, and prevent infectious, immunologic, and allergic disease.